Public Health Problem: A resurgence in HIV transmission among men who have sex with men (MSM) is a cause for concern, as the number of newly diagnosed HIV cases among MSM has increased since 1999. MSM have a considerably higher HIV prevalence rate than that of the general population, and there is growing information on recent sexual risk behavior trends among MSM, which include multiple sex partners, unprotected anal intercourse (UAI), and substance use. Objectives: The long-term objective of this research is to identify specific drug use and other high-risk behavior(s) associated with HIV infection, potentially amenable to online education, prevention and/or harm reduction. Specific Aims: The specific aims include examining the association between drug and alcohol use and the engagement in high-risk sexual behavior; testing for HIV, and knowledge of HIV status; disclosure of HIV status to (potential) sex partners among men who use the Internet to meet sex partners; and testing the viability of underlying causal pathways or processes between substance use and high-risk sexual behavior associated with HIV transmission among MSM. Research Design and Methods: We have the unique opportunity to study correlates and investigate plausible underlying causal pathways or processes of HIV risk in men who are currently being recruited for an anonymous behavioral Internet-based survey through various gay-oriented general interest and sex-oriented websites. We will explore high-risk behaviors with latent variable modeling (LVM), which examines underlying causal pathways or processes and allows evaluation of multivariable relationships (including direct and indirect effects), some of which will involve latent dimensions.